As a flow sensor for measuring the flow rate of a fluid, there is known a thermal flow sensor for sensing the flow rate of the fluid by detecting a change in electric power or a change in resistance caused by the absorption of heat of a heater by the fluid. For example, the outer periphery of a capillary having an outside diameter of about 0.5 mm is insultingly coated with a polyimide resin, and a wire of about several tens microns insultingly coated with a polyimide resin in the same way is wound on the coated capillary on the upstream side and the down stream side as a device that is part heater and part temperature sensor. By the flow of a gas in the capillary, in the device that is part heater and part temperature sensor on the upstream side, heat is absorbed by the gas flow, and the device on the downstream side is warmed by the heat carried by the gas flow. This imbalance of heat enables the sensing of flow rate. Also, in a flow sensor described in Patent Document 1, a heater is provided on the surface of a pipe conduit in which a gas flows, and an upstream-side temperature sensor consisting of a thermistor is affixed on the surface of an upstream-side pipe conduit of this heater, and a downstream-side temperature sensor consisting of a thermistor is also affixed on the surface of a downstream-side pipe conduit. The heater, the upstream-side temperature sensor, and the downstream-side temperature sensor are wire-bonded to a ceramic substrate arranged near the pipe conduit via a gold wire. Also, the ceramic substrate is provided with a plurality of pins for taking out leads so that the output signals of the temperature sensors are taken out to the outside via the pins.
Also, a flow sensor described in Patent Document 2 has a sensor pipe and an exothermic resistor in which the sensor pipe is inserted. This exothermic resistor is formed by an ordinary ceramic pipe as a base member at the outer periphery of which platinum is deposited so as to be cut off in a spiral shape by means of laser processing or trimming, so that a platinum pattern in a spiral shape is thereby formed. Also, a conductive ring is fitted at both ends of the ceramic pipe. The conductive ring plays a role of a base section to which a lead wire is connected.
Also, unlike the flow sensors described above-mentioned Patent Documents, there is known a technique in which a gas flow near a heating element is monitored visually by taking a picture of the cooling condition of a heated metal wire caused by a metal wire using an infrared camera (for example, refer to Patent Documents 3 to 6).
Patent Document 1: Japanese Patent No. 3424974 (page 2. FIG. 1)
Patent Document 2: Japanese Unexamined Patent Application Publication No. 4-366727 (pages 2 and 3, FIG. 1)
Patent Document 3: Japanese Unexamined Patent Application Publication No. 11-264769 (pages 3 and 4, FIG. 1)
Patent Document 4: Japanese Unexamined Patent Application Publication No. 2000-35438 (pages 2 and 3, FIG. 1)
Patent Document 5: Japanese Unexamined Patent Application Publication No. 2-31168 (pages 2 and 3, FIG. 1)
Patent Document 6: Japanese Unexamined Patent Application Publication No. 63-27766 (pages 1 and 2, FIG. 1)
As described above, in some cases, a heating section and a temperature detecting section consisting of a winding, a thin-film resistor, and the like are formed in a closely contacting manner on the outer wall surface of a pipe constituting a flow path. In these cases, the work generally involves difficulties, and cannot go beyond the bounds of hand making, so that the mass-producing ability and the uniformity of products are poor. In addition, the pipe constituting the flow path is often formed of a metal (electric conductor) such as stainless steel. Therefore, in order to form the temperature detecting section consisting of a winding, a thin-film resistor, and the like on the outer wall surface of the pipe in a closely contacting manner, it is necessary to interpose an insulating film therebetween. Since many of the insulating films basically have a low heat conductivity as compared with a metal, if the thickness of the insulating film is increased, the heat conduction becomes poor, and the sensitivity and response of the temperature detecting section decrease. On the other hand, if the thickness of the insulating film is decreased, there arises a problem in that the withstand voltage becomes low. Also, in some cases, the internal stress of the temperature detecting section is changed by the time change of the insulating film itself, causing an error, or the insulating film itself deteriorates and peels off the outer wall surface of pipe, disabling exact temperature measurement.
Also, in the case where a discrete temperature sensor such as a thermistor is used in the temperature detecting section, a construction is conceivable in which the temperature sensor is mechanically clamped on the outer peripheral surface of pipe conduit and is brought into close contact with the outer peripheral surface of pipe conduit by interposing a highly heat conductive grease between the temperature sensor and the outer peripheral surface of pipe conduit in place of an adhesive. In this construction, however, the grease itself may change with time, or the characteristics of the temperature sensor may change due to a stress generated in clamping, a stress caused by tightening connection between pipe couplers, and torsion of the pipe itself, so that exact temperature measurement is difficult to make.
On the other hand, since a resin-made pipe has a poor heat conductivity, even if the heating section and the temperature detecting section are formed on the outer wall surface of pipe in a closely contacting manner, accurate temperature detection cannot be carried out, so that the flow rate cannot be measured exactly.
Further, the work for taking an electrode out of the winding having an outside diameter of about several tens microns, which is formed on the outer wall surface of the capillary pipe having an outside diameter of about 0.5 mm constituting the flow path, the thin-film resistor having a pattern width of about several microns to several tens microns, and the like poses a big problem in manufacture. Careful attention must be paid to prevent the winding, the thin-film resistor, etc. from being damaged or stressed, to prevent the balance of resistance value from being lost, to prevent the heat capacity of capillary from increasing, and to prevent the heat balance from being lost.
Also, in the configuration in which the thermistor is directly affixed on the outer peripheral surface of pipe conduit as in the case of the flow sensor described in Patent Document 1, a signal must be taken out of the thermistor via wire bonding, and such a signal taking-out construction has the same problems as described above and difficulties in manufacturing.
On the other hand, even if the special conductive ring as described in Patent Document 2 is used to simplify the electrode taking-out construction, and a lead wire consisting of a platinum foil is resistance welded to the ring, the same problems as described above and the difficulties in manufacturing remain.
The flow sensor described in Patent Documents 3 to 6 merely monitors a thermal change near the heating element visually by taking a picture of the heat releasing condition and the cooling condition of the heated metal wire caused by a gas flow using an infrared camera, and does not quantitatively measure the flow rate of a fluid to be measured in the flow path formed by a particular pipe conduit.